Curriculum-Based Measurement

CBM is a specific, standardized, measurement-rigorous form of formative assessment. It uses fixed-difficulty general-outcome probes administered repeatedly so that scores are comparable over time and growth can be charted and tested against decision rules. General classroom formative assessment is broader and more flexible — questioning, feedback, and tasks embedded in teaching. CBM's distinctive contributions are reliable progress slopes and standardized decision rules. See the related Formative Assessment entry.

Because CBM measures growth, scores must be comparable across time. If probes got easier or harder as instruction moved on, a rising score could simply reflect changing difficulty rather than real improvement. By sampling probes at a constant level — typically the year-end curriculum — an increasing score reflects genuine progress toward the target, and the slope is interpretable as a rate of improvement.

Because individual probe scores are noisy, slope estimates from only a few points are unreliable. Common guidance suggests collecting on the order of several weeks of data — often cited as roughly six to ten data points or more — before drawing firm instructional conclusions, with frequent monitoring (weekly) accumulating evidence faster. Decision rules that require several consecutive points relative to the goal line guard against overreacting to a single low or high score.